Relationship Between Microstructures and Microhardness in High-Speed Friction Stir Welding of AA6005A-T6 Aluminum Hollow Extrusions

doi:10.1007/s40195-019-00970-8

Abstract

Abstract:

AA6005A-T6 aluminum hollow extrusions were friction stir welded at a fixed high welding speed of 2000 mm/min and various rotation speeds. The results showed that the heat-affected zone (HAZ) retained the similar grain structure as the base material except some grain coarsening, and the density of dislocations and β′ precipitates were almost unchanged, indicating that the high welding speed inhibited the coarsening and dissolution of β″ precipitates via fast cooling rate. The thermo-mechanically affected zone (TMAZ) was characterized by elongated and rotated grains, in which a low density of β′ precipitates and the highest density of dislocations were observed. The highest heat input and severest plastic deformation occurring in the nugget zone (NZ) resulted in the occurrence of dynamic recrystallization and a high density of dislocations. Hence, all the β″ precipitates and most of the β′ precipitates dissolved into the matrix, and a few β′ precipitates were transformed into β precipitates. The microhardness was controlled by the precipitation and solution strengthening in the HAZ, by the dislocation and precipitation strengthening in the TMAZ, and by the fine-grain and dislocation strengthening in the NZ. With the increase in rotation speed, the peak and the lowest microhardness value increased monotonously.

Key words: Aluminum hollow extrusions, High-speed friction stir welding, Grain structure, Dislocation and precipitates, Microhardness distribution

Xiang-Qian Liu, Hui-Jie Liu, Yan Yu. Relationship Between Microstructures and Microhardness in High-Speed Friction Stir Welding of AA6005A-T6 Aluminum Hollow Extrusions[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(1): 115-126.

Figures/Tables 15

Fig. 1 a Locations of the thermocouples; b comparison between the measured and predicted thermal cycles at different locations

Fig. 2 Optical pictures showing grain structures in different zones: a macrostructure of the weld cross section, b BM, c HAZ, d TMAZ, e NZ (b-e are zones taken from the black boxes in a)

Fig. 3 TEM images showing dislocations and precipitates in the BM: a dislocations, bβ′ and β precipitates at low magnification, cβ″ precipitates at high magnification, d precipitate-free zone

Fig. 4 Welding thermal cycles in different zones obtained at various rotation speeds

Fig. 5 Thermal characteristics involving peak temperature and dwelling time at high temperature in different zones obtained at various rotation speeds

Fig. 6 TEM images showing precipitates and PFZs in the HAZs obtained at different rotation speeds: a-c 2000 r/min; d-f 3000 r/min

Fig. 7 TEM images showing dislocations and precipitates in the TMAZ obtained at different rotation speeds: a-c 2000 r/min; d-f 3000 r/min

Fig. 8 Optical pictures showing grain structures in the NZs obtained at different rotation speeds: a 2000 rpm, b 2500 rpm, c 3000 rpm

Fig. 9 Average grain sizes in the NZs of the HSFSW joints obtained at different rotation speeds

Fig. 10 Dislocation structures in the NZ: a dislocation multiplication, b dislocation cells, c subgrain boundaries and dislocation walls, d grain boundaries

Fig. 11 TEM images showing dislocations and precipitates in the NZs obtained at different rotation speeds: a-b 2000 r/min; c-d 3000 r/min

Fig. 12 Relationship between microhardness distribution and microstructure characteristics

Table 1 Relationship between microstructure characteristics and microhardness

Zone	Strengthening mechanism				Microhardness
Zone	Precipitation	Solid solution	Dislocation	Fine-grain	Microhardness
BM (I)	Strongest	Weakest	Weak	-	Highest
HAZ (II)	Stronger	Weaker	Weak	-	Higher
HAZ (III)	Weaker	Weaker	Weak	-	Lowest
TMAZ (IV)	Weakest	Stronger	Strongest	-	Lower
NZ (V)	-	Strongest	Stronger	Strongest	High

Table 2 Effect of rotation speed on the changing trends of strengthening mechanisms in different zones

Rotation speed↑	Strengthening mechanism
Rotation speed↑	Precipitation	Solid solution	Dislocation	Fine-grain
HAZ	↓	↑	Same	None
TMAZ	↓	Small up	↑	None
NZ	None	Small up	↑	↓

Fig. 13 Microhardness distribution of the HSFSW joints obtained at different rotation speeds

References

[1]	X. Lu, C.S. Zhang, G.Q. Zhao, Y.J. Guan, L. Chen, A.J. Gao, Mater. Design 89, 737 (2016)
[2]	T. Kawasaki, T. Makino, K. Masai, H. Ohba, Y. Ina, E. Masakuni,JSME Int. J. 47, 502(2004)
[3]	G. Liu, L.N. Ma, Z.D. Ma, X.S. Fu, G.B. Wei, Y. Yang, T.C. Xu, W.D. Xie, X.D. Peng, Acta Metall. Sin. (Engl. Lett.) 31, 853(2018)
[4]	G.K. Padhy, C.S. Wu, S. Gao, J. Mater. Sci. Technol. 34, 1(2018)
[5]	A. Alessandro, J. Carstensen, J.F. dos Santos, J. Mater. Sci. Technol. 34, 119(2018)
[6]	J.W. Qian, J.L. Li, F. Sun, J.T. Xiong, F.S. Zhang, X. Lin, Scr. Mater. 68, 175(2013)
[7]	V. Dixit, R.S. Mishra, R.J. Lederich, R. Talwar, Sci. Technol. Weld. Join. 12, 334(2007)
[8]	P. Dong, H.M. Li, D.Q. Sun, W.B. Gong, J. Liu, Mater. Design 45, 524 (2013)
[9]	Z. Zhang, B.L. Xiao, Z.Y. Ma, Mater. Charact. 106, 255(2015)
[10]	S. Malopheyev, I. Vysotskiy, V. Kulitskiy, S. Mironov, R. Kaibyshev, Mater. Sci. Eng. A 662, 136 (2016)
[11]	A.H. Feng, D.L. Chen, Z.Y. Ma, W.Y. Ma, R.J. Song, Acta Metall. Sin. (Engl. Lett.) 27, 723(2014)
[12]	D. Rao, K. Huber, J. Heerens, J.F. dos Santos, N. Huber, Mater. Sci. Eng. A 565, 44 (2013)
[13]	H.J. Liu, X.Q. Liu, X.G. Wang, T.H. Wang, S. Yang, Int. J. Adv. Manuf. Technol. 88, 3139(2017)
[14]	X.Q. Liu, H.J. Liu, T.H. Wang, X.G. Wang, S. Yang, J. Mater. Sci. Technol. 34, 102(2018)
[15]	C.Y. Liu, B. Qu, P. Xue, Z.Y. Ma, K. Luo, M.Z. Ma, R.P. Liu, J. Mater. Sci. Technol. 34, 112(2018)
[16]	S.J. Andersen, H.W. Zandbergen, J. Jansen, C. Træholt, U. Tundal, O. Reiso, Acta Mater. 46, 3283(1998)
[17]	R. Vissers, M.A. van Huis, J. Jansen, H.W. Zandbergen, C.D. Marioara, S.J. Andersen, Acta Mater. 55, 3815(2007)
[18]	M.H. Jacobs, Philos. Mag. A 26, 1 (1972)
[19]	Y.S. Sato, M. Urata, H. Kokawa, Metall. Mater. Trans. A 33, 625 (2002)
[20]	W.C. Yang, L.P. Huang, R.R. Zhang, M.P. Wang, Z. Li, Y.L. Jia, R.S. Lei, X.F. Sheng, J. Alloys Compd. 514, 220(2012)
[21]	Y. Wang, Z.K. Liu, L.Q. Chen, C. Wolverton, Acta Mater. 55, 5934(2007)
[22]	K. Matsuda, Y. Sakaguchi, Y. Miyata, Y. Uetani, T. Sato, A. Kamio, S. Ikeno, J. Mater. Sci. 35, 179(2000)
[23]	G.A. Edwards, K. Stiller, G.L. Dunlop, M.J. Couper, Acta Mater. 46, 3893(1998)
[24]	A. Gaber, A.M. Ali, K. Matsuda, T. Kawabata, T. Yamazaki, S. Ikeno, J. Alloys Compd. 432, 149(2007)
[25]	Y. Li, L.E. Murr, J.C. McClure, Mater. Sci. Eng. A 271, 213 (1999)
[26]	C. Gallais, A. Denquin, Y. Bréchet, G. Lapasset, Mater. Sci. Eng. A 496, 77 (2008)
[27]	E.F. AboZeid, A. Gaber, Mater. Sci. Technol. 27, 487(2011)
[28]	P. Dong, Doctoral dissertation, Jilin University, 2014
[29]	D.L. Holt, J. Appl. Phys. 41, 3197(1970)
[30]	H. Mughrabi, Acta Mater. 31, 1367(1983)
[31]	W. Woo, L. Balogh, T. Ungár, H. Choo, Z.L. Feng, Mater. Sci. Eng. A 498, 308 (2008)